a. Field of the Invention
This invention pertains to antennas systems used for tracking a satellite or other source of a radio signal.
More particularly, this invention pertains to antenna systems which determine the angular position of the satellite relative to the antenna from the variation of the strength of the radio signal that is received from the satellite as the direction of the antenna is altered relative to the satellite.
b. Description of the Prior Art
In one example of the prior art, an antenna consisting of a main reflector, a subreflector and a feed was utilized to produce a "beam" of sensitivity to incident radio signals. Azimuth and elevation drive mechanisms were used to alter the angular orientation of the entire antenna structure so as to point the "beam" in a desired direction. In addition, the position of the subreflector was mechanically oscillated or "wobbled" relative to the main reflector so as to cause the beam of sensitivity to be scanned in a conical manner about the nominal, central beam position. The strength of the radio signal that was received from a satellite varied as a consequence of the conical movement of the beam and this variation in signal strength was used to determine the angular position of the satellite relative to the central beam location.
Typically, in the prior art the variation (or imbalance) in signal strength that was produced by the conical scan of the beam was "fed back" directly to the azimuth and elevation drive mechanisms so as to alter the angular orientation of the entire antenna structure in a direction that would reduce the variation in signal strength that was produced by the conical scanning of the beam about the central position. The time constants of such "feedback" systems, however, were severely limited by the tracking rates that had to be produced by the drive mechanisms in the feedback system in order to track a satellite whose angular position relative to the antenna was changing rapidly. As a consequence the feedback system had to have a relatively short time-constant in order to be able to cause the angular orientation of the antenna to change, or "slew", at a sufficiently high rate to follow or track the movement of the satellite. This short time-constant imposed significant operational restrictions upon the signal to noise ratio of the received signal that was required for successful operation of the tracking antenna.
When the antenna system is used to track a satellite whose orbital parameters are known (at least approximately), an improved prior art system has been used which utilizes the orbital parameters to predict the altitude and elevation of the satellite relative to the antenna. The altitude and azimuth of the tracking antenna are then driven in accord with the orbital predictions. The conical scan of the beam that is produced by the wobbling of the subreflector produces azimuthal and elevation error signals that are fed back respectively to the azimuth and elevation drive mechanisms to correct for errors in the prediction. If, however, the relative location of the satellite passes near the azimuthal axis of the antenna, high feedback rates, and fast responses from the drive mechanisms are required to maintain tracking.
Instead of producing a conical scan of the antenna beam about the predicted path of the satellite, another prior art antenna system has, in effect, approximated the conical scan by adding a small perturbation to the predicted values (as a function of time) of the altitude and elevation of the satellite relative to the antenna, and then sending steering commands to the drive mechanisms of the antenna in accord with these perturbed predictions. As a consequence the antenna (and its beam) was caused to scan about the predicted path in approximately a conical fashion. The variations in signal strength produced by these perturbations were then fed back respectively to the azimuth and elevation drive mechanisms. Here again, however, if the relative location of the satellite passes near the azimuthal axis of the antenna, high feedback rates, and fast responses from the drive mechanisms are required to maintain tracking. The mechanical "backlash" (sometimes referred to as "play") that is present in antenna drive mechanisms and other forces, such as wind loading caused the actual positions of the prior art antenna (and the antenna beam) to deviate slightly from the positions specified by the steering commands, which deviations degraded the operation of the feedback system.